The present invention relates to a catalytic reaction for reduction of nitrogen oxide (NOx) in exhaust gas. More particularly it relates to said catalytic reaction in which a plurality of tubular catalyst units are arranged in the cross-section of the flow path of a gas flowing thereinto.
Generally, in the heterogeneous gas phase reaction employing a solid catalyst, it is necessary to take the following matters into consideration in order to obtain the highest reaction efficiency using the least amount of catalyst:
(1) The catalyst activity per apparent surface area of the catalyst should be increased.
(2) The apparent surface area of catalyst per unit area thereof should be increased.
As for the methods for achieving the above matters,
(i) the shape of catalyst should be devised; and PA1 (ii) the characteristic particle diameter of catalyst should be minimized as allowable.
(3) The whole of apparent surface of catalyst should be effectively utilized.
Among these three important matters, the item (1) relates to the basic characteristic of catalyst, and concerns the problem of the catalyst itself. On the other hand, the items (2) and (3) relate to how effectively to use such a catalyst, and are important particularly when the catalyst is commercially employed.
Heretofore, in the heterogeneous gas phase reaction, for example, in the sulfuric acid production process using sulfur dioxide converter, the above-mentioned item (2)-(ii) is emphasized, with the result that pellets having as relatively small a size as about 5 mm in diameter and about 10 mm in length have been employed as the catalyst units therefor.
However, in case of denitration of combustion exhaust gases, particularly in case of dry catalytic reduction process therefor, a large amount of powder dust and soot dust, both of which will be hereinafter referred to as dusts, is entrained and deposited on the catalyst surface.
In the above case, if a catalyst unit having a small size as mentioned above is employed, there is raised a problem of masking the surface of the catalyst unit and resultantly clogging the packed catalyst unit bed with dusts. Thus, its solution has been directed to modifying a catalytic reactor. An apparatus for moving bed type, an apparatus for parallel contact of gas flow with catalyst or the like has been proposed, which, however, has resulted in more complicated apparatus and higher cost equipment.
Recently, in order to solve the above-mentioned problem of clogging catalyst unit bed with dusts, a large size of catalyst unit having a large characteristic particle diameter has begun to be employed. As for the solid catalyst employed for the denitration of exhaust gases, particularly for the dry catalytic reduction process therefor, a large size, tubular catalyst unit is being employed, and its dimension is such that the diameter thereof is about scores of mm and the height is about scores to several hundreds mm. Such large size, tubular catalyst units are packed inside a reaction vessel in a stacked manner, and in contact with one another in the cross-section of the flow path of a gas inside a reaction vessel, with the axes thereof being arranged in accord with the flow direction of the gas.
As for the method for arranging such tubular catalyst units, there are exemplified a square pitch arrangement (Japanese Utility Model Publication No. 10455/1975) and a triangular pitch arrangement. In these arrangements, the units are usually stacked concentrically. These arrangements are shown in FIG. 1 and FIG. 2 among the accompanying drawings. FIG. 1 and FIG. 2 show the cross-sectional views of the tubular catalyst units in the case of a square pitch arrangement and in the case of triangular pitch arrangement, respectively. In these figures, reference numeral 1 indicates a tubular catalyst unit.
Since there is a large difference in the flow rate of gas between the flow path through the inside of the tubular catalyst unit (Si in the figures) and the flow path through the outside thereof (Se or Se' in the figures), it has become apparent that various problems are raised.
Assuming that the outer diameter of the tubular catalyst unit is D.sub.o, the equivalent diameter of the flow path formed outside the units is represented as 0.27 D.sub.o in case of the square pitch arrangement and 0.10 D.sub.o in case of the triangular pitch arrangement. The equivalent diameter referred to herein is defined as a value obtained by dividing 4 times the cross-sectional area of the flow path formed by the tubular catalyst units, by their wetted perimeters. In this connection, the equivalent diameter in case of the square pitch arrangement, 4R.sub.He, will be given by the following equation: ##EQU1##
According to the knowledges of hydrodynamics, the ratio of the flow rate of gas inside the tubular catalyst unit to that of gas outside the unit, Q.sub.si /Q.sub.se is given by the following equation: ##EQU2## wherein 0.5.ltoreq.m.ltoreq.2.0; Si and Se represent the cross-sectional areas of the flow paths inside and outside the tubular catalyst unit, respectively; Vi and Ve represent the gas flow rates inside and outside the tubular catalyst unit, respectively; Di represents the inner diameter of the tubular catalyst unit, and 4R.sub.He represents the equivalent diameter of the cross-sectional area of the flow path formed outside the catalyst unit.
As mentioned above, in case of the square pitch arrangement, since 4R.sub.He is 0.27 D.sub.o, Q.sub.si becomes larger than Q.sub.se, and this tendency becomes more evident with the decrease in the thickness of the tubular catalyst unit. Taking into consideration the fact that the heterogeneous gas phase reaction occurs on the surface of catalyst units, the flow rates of gas through the inside and outside flow paths, per respective surface areas of the catalyst units will be compared as follows.
The flow rates of gas per the surface area of the tubular catalyst unit, Q.sub.si /A.sub.si or Q.sub.se /A.sub.se, will be given by the following equations: EQU Q.sub.si /A.sub.si =Q.sub.si /D.sub.i .pi.l EQU Q.sub.se /A.sub.se =Q.sub.se /D.sub.o .pi.l
wherein l represents the height of the packed catalyst bed, and A.sub.si and A.sub.se represent the surface areas in the flow paths inside and outside the tubular catalyst units, respectively.
In this case, since A.sub.se is apparently larger than A.sub.si and Q.sub.si /A.sub.si is further larger than Q.sub.se /A.sub.se, the flow rate of gas per unit surface area outside the tubular catalyst units will become very small. Thus, it has become evident that the outer surface of the unit is not effectively utilized.
Further, because of the fact that the flow rate of gas outside the tubular catalyst units becomes smaller, masking of the catalyst surface or clogging of the space outside the units, due to dusts, is liable to occur, which is a serious problem, particularly in case where an exhaust gas including a large amount of dust is treated.
The characteristic particle diameter of the catalyst units necessary for avoiding such masking of catalyst or clogging due to dusts is determined based upon the flow path formed outside the tubular catalyst units, since the cross-sectional area of the flow path formed outside the units is smaller than that of the flow path formed inside the units. Thus, in order to prevent such masking of catalyst or clogging, it may be devised to increase the characteristic particle diameter of the catalyst units to such an extent that the masking or clogging does not occur in the flow path formed outside the tubular catalyst units. In this case, however, the flow path formed inside the units will become further larger than that required for preventing the masking or clogging.
On the other hand, in order to maintain a higher percentage conversion of gas, it is necessary to make use of the catalyst surface more effectively. Thus, a method wherein the surface area of the packed catalyst units per unit volume thereof and the catalyst surface area per unit weight thereof are made larger, i.e. a method wherein the characteristic particle diameter of the catalyst units is made as small as possible, will be advantageous.
In addition, with a fixed bed type catalytic reactor, heretofore it has been known that, identical in surface area outside and inside thereof, a catalyst having honeycomb structure is of a reasonable shape. However, with the catalysts made of such material as TiO.sub.2 -V.sub.2 O.sub.5 and Al.sub.2 O.sub.3 -TiO.sub.2 which are filled in a reactor for reducing and removing NO.sub.x from the exhaust gas from a combustion equipment such as a boiler, refuse incinerator, cintering furnace, blast furnace and sludge incinerator, there has been encountered a problem of strength, and hence, there has not been manufactured a honeycomb shaped catalyst. Small honeycomb shaped catalyst are used for denitrating the exhaust gas from motor cars. However, large honeycomb shaped catalysts are not to be used for denitrating a large amount of exhaust gas (from scores of thousands to hundreds of thousands m.sup.3 /hour) exhausted from the combustion equipment described above. Much labor being required for filling in the reactor, small honeycomb shaped catalysts are impracticable for the combustion plants described above.
One object of the present invention is to provide a catalytic reactor for reducing nitrogen oxide (NO.sub.x) contained in exhaust gas, wherein inner and outer surfaces of tubular catalyst units are effectively utilized, the catalyst units are protected from being clogged by dust from outside thereof, and packed catalyst units in a reaction vessel can be reduced in weight or volume.
The present invention is intended to provide a catalytic reactor for reducing nitrogen oxide (NO.sub.x) contained in exhaust gas, wherein a multiplicity of tubular catalyst units each being a ring shaped in cross-section are disposed in the cross-section of the flow path of the exhaust gas in such a manner that tubular axes thereof are in parallel with the direction of gas flow within a reaction vessel through which the exhaust gas containing nitrogen oxide (NO.sub.x) flows, constructed such that said catalyst units are disposed in spaced apart relationship with one another so that the respective equivalent diameter of the cross-sectional areas of the flow pass of gas formed inside and outside the tubular catalyst units per pitch of the tubular catalyst units can be made nearly equal and said catalyst units are each secured to the interior of reaction vessel at least at two points in the longitudinal direction by support means.